/*
 * Copyright (c) 2013-2015, ARM Limited and Contributors. All rights reserved.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <arch_helpers.h>
#include <assert.h>
#include <bakery_lock.h>
#include <cpu_data.h>
#include <platform.h>
#include <string.h>

/*
 * Functions in this file implement Bakery Algorithm for mutual exclusion with the
 * bakery lock data structures in coherent memory.
 *
 * ARM architecture offers a family of exclusive access instructions to
 * efficiently implement mutual exclusion with hardware support. However, as
 * well as depending on external hardware, the these instructions have defined
 * behavior only on certain memory types (cacheable and Normal memory in
 * particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases
 * in trusted firmware are such that mutual exclusion implementation cannot
 * expect that accesses to the lock have the specific type required by the
 * architecture for these primitives to function (for example, not all
 * contenders may have address translation enabled).
 *
 * This implementation does not use mutual exclusion primitives. It expects
 * memory regions where the locks reside to be fully ordered and coherent
 * (either by disabling address translation, or by assigning proper attributes
 * when translation is enabled).
 *
 * Note that the ARM architecture guarantees single-copy atomicity for aligned
 * accesses regardless of status of address translation.
 */

#define assert_bakery_entry_valid(entry, bakery) do {    \
    assert(bakery);                    \
    assert(entry < BAKERY_LOCK_MAX_CPUS);        \
} while (0)

/* Obtain a ticket for a given CPU */
static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me)
{
    unsigned int my_ticket, their_ticket;
    unsigned int they;

    /* Prevent recursive acquisition */
    assert(!bakery_ticket_number(bakery->lock_data[me]));

    /*
     * Flag that we're busy getting our ticket. All CPUs are iterated in the
     * order of their ordinal position to decide the maximum ticket value
     * observed so far. Our priority is set to be greater than the maximum
     * observed priority
     *
     * Note that it's possible that more than one contender gets the same
     * ticket value. That's OK as the lock is acquired based on the priority
     * value, not the ticket value alone.
     */
    my_ticket = 0;
    bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket);
    for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) {
        their_ticket = bakery_ticket_number(bakery->lock_data[they]);
        if (their_ticket > my_ticket)
            my_ticket = their_ticket;
    }

    /*
     * Compute ticket; then signal to other contenders waiting for us to
     * finish calculating our ticket value that we're done
     */
    ++my_ticket;
    bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket);

    return my_ticket;
}


/*
 * Acquire bakery lock
 *
 * Contending CPUs need first obtain a non-zero ticket and then calculate
 * priority value. A contending CPU iterate over all other CPUs in the platform,
 * which may be contending for the same lock, in the order of their ordinal
 * position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket
 * (and priority) value as 0. The contending CPU compares its priority with that
 * of others'. The CPU with the highest priority (lowest numerical value)
 * acquires the lock
 */
void bakery_lock_get(bakery_lock_t *bakery)
{
    unsigned int they, me;
    unsigned int my_ticket, my_prio, their_ticket;
    unsigned int their_bakery_data;

    me = plat_my_core_pos();

    assert_bakery_entry_valid(me, bakery);

    /* Get a ticket */
    my_ticket = bakery_get_ticket(bakery, me);

    /*
     * Now that we got our ticket, compute our priority value, then compare
     * with that of others, and proceed to acquire the lock
     */
    my_prio = PRIORITY(my_ticket, me);
    for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) {
        if (me == they)
            continue;

        /* Wait for the contender to get their ticket */
        do {
            their_bakery_data = bakery->lock_data[they];
        } while (bakery_is_choosing(their_bakery_data));

        /*
         * If the other party is a contender, they'll have non-zero
         * (valid) ticket value. If they do, compare priorities
         */
        their_ticket = bakery_ticket_number(their_bakery_data);
        if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) {
            /*
             * They have higher priority (lower value). Wait for
             * their ticket value to change (either release the lock
             * to have it dropped to 0; or drop and probably content
             * again for the same lock to have an even higher value)
             */
            do {
                wfe();
            } while (their_ticket ==
                bakery_ticket_number(bakery->lock_data[they]));
        }
    }
    /* Lock acquired */
}


/* Release the lock and signal contenders */
void bakery_lock_release(bakery_lock_t *bakery)
{
    unsigned int me = plat_my_core_pos();

    assert_bakery_entry_valid(me, bakery);
    assert(bakery_ticket_number(bakery->lock_data[me]));

    /*
     * Release lock by resetting ticket. Then signal other
     * waiting contenders
     */
    bakery->lock_data[me] = 0;
    dsb();
    sev();
}
